A leading cause of blindness in developed countries is Age-related Macular Degeneration (AMD). AMD is a disease which degrades the photoreceptors (i.e., rods and cones) in the retina, but generally does not degrade other parts of the retina. Accordingly, one approach for treating AMD (and other conditions, such as retinitis pigmentosa, that also primarily degrade the photoreceptors) is to implant a retinal prosthesis providing the same functionality as healthy photoreceptors (i.e., selective stimulation of neural tissue in the retina responsive to visual inputs).
Such a retinal prosthesis performs the functions of 1) receiving a visual image and 2) selectively stimulating retinal neural tissue responsive to the received visual image. In many cases, both of these functions are performed electrically, since electrical detection of optical signals and electrical excitation of neural cells are both known. However, a known problem in the art is providing sufficient electrical power to such a prosthesis, as the following example will make clear.
An electrode for stimulating a neural cell typically has a radius ro of about 5 μm. The impedance between an electrode of radius ro and a (hypothetical) large electrode at infinity in a medium having resistivity γ is given by R=γ/(2πro). For this example, we assume a resistivity γ=70 Ωcm, which is the resistivity of a typical saline solution and is thus representative of the resistivity to be expected in vivo. Thus the resistance R seen by the electrode of this example is 22 kΩ.
The electrode potential U (relative to infinity) required to establish a potential drop ΔU across a cell of length L in contact with the electrode is given by U=ΔU(ro+L)/L. Assuming a cell length L=10 μm, and a typical cell depolarization voltage of 30 mV, a potential U=45 mV and current I=2 μA is required on the electrode to depolarize the cell in this example. The power dissipation P=I2R=U2/R in this example is about 0.35 μW while the current is flowing. As the electrode size ro increases, the required power increases because R decreases and U increases for fixed ΔU.
The power flux of ambient light on the retina is typically about 0.9 μW/mm2 outdoors on a sunny day. The conversion efficiency of optical power to electrical power provided by photovoltaic cells is on the order of 30%, so the available electrical power at the retina is roughly 0.3 μW/mm2. The area of the macula is about 9 mm2, so a retinal prosthesis powered by a macula-sized photocell would only have enough power to drive about 8 electrodes, which is far fewer electrodes than what is needed to provide reasonable vision for a patient.
For this reason, power is typically supplied to an electrical retinal prosthesis externally. Known methods for accomplishing this include wireless transmission of radio-frequency (RF) power to an RF receiver incorporated into the intra-ocular portion of the prosthesis, and transmission of optical power to an intra-ocular photovoltaic cell connected to the prosthesis. The source for optical power transmission is typically a laser attached to glasses or goggles worn by a patient. Examples of such approaches are given in U.S. Pat. No. 6,324,429 to Shire et al., U.S. Pat. No. 6,298,270 to Nisch et al., and U.S. Pat. No. 4,628,933 to Michelson. In some of these prior art approaches, the external unit of the retinal prosthesis provides both electrical power and electrical image data to the intra-ocular unit of the prosthesis. In other cases, the external unit only provides power, and the intra-ocular unit converts the optical image to electrical form to drive neural cells in the retina. In either case, such prior art retinal prostheses require an external unit to at least provide power to the intra-ocular unit.
Accordingly, there is a need for a fully autonomous electrical retinal prosthesis requiring no external unit, which is not met by prior art approaches.